1. Field of the Invention
The present invention relates to an improvement in a transmission, particularly a five-speed transmission for an automobile.
2. Description of the Prior Art
With most of the manual transmissions for automobiles hitherto used, on a main shaft serving as an output shaft journaled in a gear housing are loosely mounted various speed-change gears among which one gear corresponding to a required vehicle speed is selected and fixed to the main shaft by a synchromesh device or the like. With a transmission including overdrive gearing, however, it has recently been suggested to loosely mount one overdrive gear on the counter shaft and fix the other overdrive gear onto the main shaft for the purpose of decreasing the moment of inertia of the overdrive gears and obtaining higher vehicle speeds by reducing the diameter of the overdrive gear.
As shown in FIG. 1, with such a transmission, which is shown as a five-speed version for a passenger car, a main shaft 5 is journaled in bearings 9, 11 and 14b. Bearing 9 is located in a front wall of a transmission casing 1, bearing 11 is located in an adaptor plate 1b provided between the transmission casing 1 and rear extension 3 and bearing 14b is located in a stiffener plate 14a of the rear extension 3. First to fourth gears 17, 16 15 and 13 are mounted on the main shaft 5 between the bearings 9 and 11, and a reverse gear 28 and an overdrive gear 20 are arranged between the bearings 11 and 14b.
In this case, the main shaft 5 between the bearings 9 and 11 has a maximum diameter at its mid portion where it is subjected to maximum loads and stepwise progressively reduced diameters towards ends in consideration of the rigidity of the shaft. Therefore, the diameter of the main shaft where the heavier first gear 17 is mounted is small, so that the first gear is arranged adjacent to the bearing 11 as near as possible to reduce a moment arm, thereby avoiding an excess stress acting upon the main shaft.
In a transmission for a truck or the like it is possible to form an overdrive gear integrally with a main shaft by increasing its diameter because of no limitation of a space for the shaft. In a transmission for a passenger car, however, such an increase in diameter of a main shaft has a limitation in space and makes it difficult to pass the main shaft through a transmission casing and to arrange various gears for assembling the transmission and actually impossible unless an entire dimension is changed.
It may be possible to reduce the size of the overdrive gear by reducing the number of its teeth. However, the number of teeth of a gear has a minimum limitation under a determined module. Accordingly, the gear ratio is limited under a determined length of the main shaft and the limited gear ratio obstructs the improvement of the performance of the transmission. In this case, the overdrive gear has been usually formed as a ring gear which is removably fixed onto the main shaft by means of spline grooves and stoppers which would require extra cutting processes and an increase in the number of parts.
In such hitherto used five-speed transmissions, on the other hand, as the main shaft is journaled by the three bearings, the alignment of these bearings with a high degree of accuracy is difficult and the elongated main shaft makes it difficult to render the transmission compact.
To avoid this, it has been suggested to arrange the reverse gear 28 and the overdrive gear 20 within the transmission casing 1 and to support the main shaft in only two bearings 9 and 11. With this arrangement, however, the span between the bearings is elongated, and the heavy first gear 17 is located on the relatively small diameter portion of the main shaft intermediate the bearings 9 and 11. This seriously compromises the rigidity of the main shaft.
Moreover, as shown in FIG. 2 the main shaft is formed at substantially its mid portion with a large diameter flange 50 to axially locate the gears and synchromesh devices, and is formed with journals 36, 37, 39, 40 and 38 whose diameters reduce toward the ends in a stepwise manner.
In cutting and grinding the main shaft, therefore, machining must be effected using the flange as a reference line, which makes machining difficult because longitudinal cutting or grinding in a single operation is impossible. During subsequent assembly, the change gears and synchromesh devices must be mounted on the main shaft from its ends, so that the main shaft cannot be incorporated in the transmission until all mountings of the gears and synchromesh devices have been completed. In addition, as the second and third gears 16 and 15 having intermediate outer diameters are unavoidably mounted on enlarged diameter portions 37 and 36 of the main shaft, radial thicknesses of these gears are decreased, thus also comprising the strength of the gears. This holds true in the synchromesh device and particularly the synchronizing hub.